It's been a while, but I've completed the stringer replacement project. I used one of the old ones as a template and cut two new stringers. I shaped them with a belt sander until they fit almost perfectly, coated the bottoms and the ends with resin for protection from water seapage, gave it enough time to harden, and then dropped them into place. Before I did, though, I scrubbed the hell out of the bilge area. This was not only long over due, but necessary to allow the fiberglass and paint to stick later. I measured about twenty times and adjusted the stringers until they were perfectly centered. Then I cut a couple of spacers to hold them the right distance apart. From the other side of the first bulkhead I drove a couple of screws into the end of the stringers to hold them in place.
Then the first batch of fiberglass went on. We did the ends first, again to help hold them in place. Then we laid a long piece that wrapped the entire stringer and extended about 6 inches on both sides.
It took several sessions with drying and grinding time in between, but the result was pretty good for a couple of amateurs! We put some reinforcement on both ends and along the top of the stringer, and two strips of heavy weave along the inside of each stringer right at the point where the motor will be mounted. We're not sure if there's any structural benefit to doing this, but what the hell. We had some extra materials!
I sanded the entire bilge area so that I would have a good surface for the "Bilgekote" paint that I bought at West Marine. I applied two coats of bright white (an upgrade over the grey that I used to have), and it turned out great!
It took a long time, but I'm glad I did this. Since the motor was out anyway, it was a really good opportunity, and it was definitely needed.